Waterbased inkjet inks for thermal and piezo inkjet print heads in home-office applications have been prevalent for decades, but are characterised by slow drying and poor resistance qualities. However, in industrial print applications involving multiple print-head arrays, UV cured inkjet inks are prevalent, where fast drying and high resistance properties are a key benefit. However, despite their numerous advantages, UV curable inkjet inks are well known to have problems due to the relatively high film thickness applied (e.g. 10-12 microns), which causes problems to the printer as a result of an uneven ink build across the web and distortion of the substrate roll in the machine. It is also well known that a potential solution to this problem is the use of a waterbased UV curable ink, which initially deposits a similar film thickness to 100% UV inks (e.g. 10-12 microns), but when the inks have been dried prior to UV curing, the film thickness is reduced to only a few microns, and therefore drastically limits the roll distortion whilst maintaining many of the positive product resistance properties associated with UV curable inks.
The viscosity requirements for industrial print applications vary by print head manufacturer, depending on design principles and the flow requirements within the print head channels. For single pass systems, requirements tend to be lower (6 mPas or less) due to the higher degree of print head integration that is required to achieve higher nozzle density in two-dimensional arrays utilising thin-film actuators, as exemplified by Dimatix Samba and the Kyocera KJB-Z. In graphics applications, print heads are commonly based on bulk piezoelectric actuators in linear arrays and the typically larger drop size regimes tend to lead to higher optimal viscosities of 8 mPas and above to achieve good drop formation and desired level of jetting reliability. The Dimatix Q-class designs and Ricoh MH print head families are examples of this, although Xaar and other chevron actuator heads such as Konica Minolta, Toshiba Tec and SII Printek also are in this range.
One significant limitation of aqueous UV technology is that the high levels of water that are desirable in the formulation lead to a particularly low ink viscosity (typically 2-3 mPas at 32° C.). Adjusting the ink to achieve an acceptable print viscosity in commercial inkjet heads (typically 4-9 mPas at 32° C.) then represents a significant problem, since it is difficult to tailor a single formulating strategy to cover such a wide range of application viscosities. Several approaches to increasing the viscosity would be easily known to those skilled in the art, but these have consequential problems which may not necessarily be problematic in a pure water-based inkjet ink. But in a water-based UV ink where final product resistance characteristics are a key measure of performance, they are of enormous significance. Examples of some techniques for increasing viscosity, and the problems associated therewith include:    1. Increased co-solvent levels. This can significantly increase the viscosity of the ink formulation, but care needs to be taken because of the deleterious effect this then has on the drying performance of the ink, where significant extra expense and space is required to accommodate increased drying capacity of the equipment. As well as increased solvent emission concerns, in many cases, the solvents can also have a negative impact on the stability of the pigment dispersion.    2. Increased resin content. The typical resin used in these formulations is an aqueous polyurethane dispersion. Polyurethane dispersions themselves are low viscosity and have limited viscosity altering impact on the formulation. It has also been demonstrated that jetting performance is adversely affected at higher solids levels.    3. Water-soluble polymers. Materials commonly used by those skilled in the art would include things like polyethylene glycol. Whilst this type of material is somewhat effective at increasing viscosity, in order to achieve the several mPas increase in viscosity required, 5-10% addition of these materials is often necessary, which can significantly impair the physical properties of the dried and cured ink, since, after drying, they may make up as much as 30-50% of the dried ink film. Other water-soluble polymers such as polyvinylpyrrolidone can also be used, but can also seriously affect the UV cure properties as well as the jetting efficiency.    4. Reduce jetting temperature. UV curable inkjet inks are typically jetted at a temperature of 40-50° C. but water-based and water-based UV are already typically jetted at a lower temperature of around 32° C., and further reduction in temperature would have implications on the design of inkjet heads in order to avoid condensation from a necessary cooling circuit.
Rheology modifiers are organic or inorganic coating additives that control the rheological characteristics of the liquid formulation. In coatings technology, rheology modifiers are mainly used to provide either pseudoplastic or thixotropic properties. These can be divided into inorganic and organic materials; inorganic additives are typically clays, and fumed silicas, whereas organic materials can be subdivided into natural materials such as cellulosics/xanthan gum and synthetic materials which are then associative or non-associative type materials.
Inorganic rheology modifiers are typically dispersed into a coating and function as suspended or gelling agents. Usually the viscosity of the formulation decreases with time and the constant shear conditions as its gel structure is broken down. If this shear is removed, the coating gradually recovers to its original viscosity. Inorganic rheology modifiers are sometimes added to aqueous formulations as secondary thickeners to improve the anti-sag, anti-settling, anti-synerisis and anti-spattering properties of the coating.
Organic rheology modifiers are more diverse in nature and subdivide into many structural types. Non-associative rheology modifiers act by entanglement of soluble, high molecular weight polymer chains and thus their effectiveness is mainly controlled by the molecular weight. These tend to have pseudoplastic rheology, giving good stabilisation against settling and sagging, and therefore find significant use in non-drip gloss type paint applications. Associative thickeners function by non-specific interactions of hydrophobic end groups with both themselves and components of the coating. Thus, they form a physical network. According to the BYK technical information leaflet Optiflo—water-soluble, associative thickener for aqueous formulations; “associative thickeners greatly increase viscosity, especially under high shear conditions. This strengthens the internal structure, thickens the material to a honey-like consistency, a process also described as brush drag”.
Thickeners typically used for water-based systems include cellulosics, acrylic thickeners (alkali swellable emulsions ASE, hydrophobically modified alkali swellable emulsions HASE), hydrophobically modified polyurethanes HEUR, hydrophobically modified polyethers HMPE, and specialty clays.
Although some suppliers characterise thickeners in terms of their degree of Newtonian behaviour, it would be appreciated by those skilled in the art that this is still a relative concept for a rheology modifier whose primary purpose is to improve levelling, anti-spatter and brush/roll application performance. As such, one of ordinary skill in the art would expect that a Newtonian fluid would not have any impact on performance for these criteria and that all thickeners have the purpose of imparting non-Newtonian rheology modification.
Natural and synthetic rheology modifying agents have been used in water-based, non-energy curable inks. For example, EP 1464686, US 2010/041816, WO 2011/079402, WO 2009/104042, and WO 2015/152862, all disclose water-based inks comprising rheology modifying agents. However, the inks described are not energy curable.
EP 1464686 discloses an ink for a ballpoint pen which has high viscosity at low shear rates to stop the ink flowing during writing pauses, and low viscosity at high shear rates to ensure suitable ink feeding during fast writing.
EP 2184329 describes an ink for inkjet or screenprinting which is based on a water diluted UV curable resin and optionally contains a thickener. The thickeners described are sodium alginate, sodium carboxymethylcellulose or polyurethane solution. However, the patent only describes and claims the use of such thickeners in a version of the invention which is the screen ink formulation, where pseudoplastic behaviour is desirable and is a general characteristic of all screen inks. The patent does not disclose the use of such thickeners in an inkjet ink.
US 2010/0041816 describes water-based inkjet inks which contain a water-soluble polymer thickener. The water-soluble thickener is selected from the group consisting of polyvinyl alcohol, polyvinylpyrrolidone, polyoxyethylene glycol or a polyoxyethylene/poly oxypropylene block polymer. A number of these materials are included in the examples but the use of thickeners as described in this invention is not disclosed.
Note that the use of polyvinyl alcohols was discounted within the UV curable water-based formulations of the present invention because they were either insoluble, unstable or had a significant deleterious effect on jetting performance. Polyoxyethylene/polyoxypropylene block copolymers were also investigated within the present invention and found to be satisfactory from the point of view of ability to increase viscosity, but the levels required to achieve viscosity targets determined by the print head were such that there was a significant negative effect on the performance properties of the dried and cured ink.
EP 2703458 discloses a photocurable ink composition for aqueous UV curing inkjet applications containing a water-based polyurethane dispersion, water, photoinitiator, colorant and hydrophobic radiation curable monomers. The application describes the optional use of several additives, including rheology modifiers. But there is no reduction to practice for this, with no examples containing any rheology modifiers.
Lamont, et al. describe the use of inkjet inks with added 1% polystyrene solution as rheology modifiers to provide a nearly twofold gain in viscosity, with the largest viscosity gains coming from the polymer with the highest molecular weight. However, these high molecular weight additives gave reduced jetting performance and solvent compatibility issues. Such an approach with a largely water-based ink of the present invention would be wholly unsatisfactory for reasons of compatibility. Organic electronics (2015), 17, 107-114—tuning the viscosity of halogen free bulk hetero junction inks for inkjet printed organic solar cells (SciFinder abstract).
WO 2011/079402 describes an aqueous ink set which is relatively high viscosity prior to printing and just after printing but which shear thins as it passes through the head of the inkjet printer in response to shear stress. The inkjet ink is not Newtonian in character. In addition, these inks are not suitable for UV curing.
WO 02/066571 discloses energy curable pressure sensitive adhesives that contain a thickening or thixotropic additive. The adhesives are in an initial gel state, but change to a low viscosity fluid when subjected to a threshold of suitable energy (e.g. shear, thermal, sonic). Many different rheology modifiers are disclosed, with surface treated inorganic oxide particles being preferred.
Thus an unsolved problem exists, which is to create small absolute, but large relative increases in viscosity of water-based and UV curable inkjet inks, without negatively impacting performance. This may be done via the addition of a very low level of an additive that itself preferably imparts minimal change to the drying, curing and physical properties of the ink once dried and UV cured, and allows a fundamentally similar product formulation strategy to be tailored for use across a wide number of different print-heads.